1. Field of the Invention
The invention relates to an image pick-up apparatus and a method for picking up an image and, more particularly, to an image pick-up apparatus which, when the finger etc., is put on a certain place, automatically picks up its skin marking pattern to provide a digital image.
2. Description of the Related Art
As a system for identifying an individual by his fingerprint there is available, an image matching apparatus that automatically checks an identity existing fingerprint information just input and a previously registered one, using image processing and pattern recognizing technologies. This type of the image matching apparatus has already been proposed in many cases such as in Japanese Examined Patent Publication (KOKOKU) No. 63-21233.
Conventionally, this type of the image matching apparatus has a fingerprint input unit for generating digital image of a fingerprint mark for the finger put on a loading surface made of a transparent body such as glass, provided with a light source, a Charge Coupled Device (CCD), etc., by utilizing the optical boundary conditions of glass with applying a light on the back face of that loading surface. Such a fingerprint input unit is disclosed in, for example, the Japanese Examined Patent Publication (KOKAI) No. 5-225345.
Also, there has recently been put to practical use such a device that is capable of a sensing approach to such finger print mark other than the optical one. This device senses irregularities such as convexed and concaved portions of a fingerprint put into directly contact with the surface of a semiconductor chip, for example, a type of detecting same with a static capacitance or a type of detecting same utilizing a difference in temperature or electric field.
In such a past fingerprint input unit, the image of a fingerprint thus input is liable to have a superiority fluctuation in picture quality caused by a fluctuation in the finger pressing state. Since this superiority fluctuation in picture quality greatly effects on the subsequent matching processing, it is desired to pick up an appropriate fingerprint image data from images picked up when the finger is pressed on that surface.
It is also desirable for user interface to, when picking up a fingerprint image information, automatically pick up an appropriate image without indicating the timing but only by putting the finger on the apparatus.
The conventional image pick-up apparatus has employed such a method that, to automatically pick up an appropriate images from the input unit for example, will extract an image when the apparatus decides the number of pixels thereof at existing a predetermined position of the image taken from the loading surface, each of which having a gray level value not less than a predetermined threshold of the gray level value Td is not less than a predetermined threshold number of pixels, Tn.
This method can automatically extract appropriate fingerprint images among the plurality of such images by using an appropriate threshold gray level value and an appropriate threshold number of pixels, when a distribution of the gray level value density within one fingerprint image is nearly uniform and a fluctuation in size of line pattern of the finger print is small within one fingerprint image.
This method, however, has such a problem that a fingerprint image cannot automatically be input because the number of pixels each having the respective gray level value exceeding over a predetermined gray level value is reduced when the gray level value of the pixels consisting a part of the image within the same fingerprint image data is caused to be reduced due to a scar or the like, or when the size of line pattern of the finger print of the fingerprint image data is narrow.
Conversely, in order to cope with this problems, when the threshold of the gray level value Td or the threshold number of pixels Tn is reduced, the number of pixels each having the gray level value exceeding over a predetermined threshold of the gray level value, becomes larger than a predetermined threshold number of pixels for such a fingerprint image that has a large width size of line pattern of the finger print pattern, even when the finger is not completely contacted with the plated, thus causing fingerprint images to be picked up before a sufficient picture quality is comes out.
Further in the conventional method, the threshold gray-level value used for detecting such a line pattern of the finger print, is fixed and thus when the line pattern of the finger print of a finger being contacted with a surface of a place on which the finger to be detected is mounted, is narrow, a number of pixels each having the gray-level value exceeding over a predetermined threshold gray-level value thereby even a fingerprint image which having clear image about the line pattern of the finger print, could not be selected automatically.
While, inversely, when the line pattern of the finger print of a finger to be detected is thick, a number of pixels each having the gray-level value exceeding over a predetermined threshold gray-level value, exceeds over a predetermined threshold value of the pixel numbers even when the finger to be detected did not completely come into contacted with the place on which the finger to be detected is mounted, thereby the fingerprint image could be selected before the quality of the fingerprint image becomes sufficient causing a problem in that a control for selecting such image information becomes difficult.
From these point of views, it is desired to provide a method of automatically picking up appropriate fingerprint images even when there are large fluctuations in the gray level value distribution or the size of line pattern of the finger print.
In addition to this, another object of the present invention is to provide an apparatus of and a method of automatically picking up appropriate fingerprint images having better quality thereof by analysing an inputting condition of a finger depending upon a chronological change information in a number of pixels the gray-level value of which being chronologically increased or decreased so as to resolve the above-mentioned drawbacks in the conventional apparatus.